164 



thin film of glass into the eye and apparently 

 projected through the glass upon the paper. 



Another simple form is the Wollaston camera 

 lucida (Fig. 43), which is attached to the eyepiece 

 in the same manner as the foregoing, with the 



Fig. 43. 



instrument also inclined. This consists of a quad- 

 rangular prism, which reflects the emergent pencil 

 through a small opening into the eye. The eye is 

 placed over the edge of the prism in such a man- 

 ner that the rays forming the image enter only a 

 portion of the pupil, while its other portion views 

 the paper. In using this however, the eye must 

 be kept quite steadily in the proper position. 



While there are some other forms, they are not 

 in general use. The very best is one designed by 

 Prof. Abbe, and which goes under the name of the 

 Abbe camera lucida (Fig. 44). It is fixed to the 

 tube of the microscope, permitting the mounting 

 which contains the revolving prism to be swung 



